Water ordinarily freezes at 0 degrees Celsius or 32 degrees Fahrenheit. In a system at this temperature, the rate of freezing is the same as the rate of melting. This is known in chemistry as a dynamic equilibrium. Any change in the temperature will change the rates of freezing and melting, and the entire system will freeze or thaw. Adding salt changes these rates but without changing the temperature. The salt causes the rate of freezing to occur slower than the rate of the rate of thawing, causing the ice to melt and requiring a lower temperature to reattain equilibrium.
In a dynamic equilibrium, different reactions occur at the same rate. Imagine an ice cube in a bowl of water. If the bowl is at room temperature, the ice will melt. This is because the rate of melting is faster than the rate of freezing. Conversely, if you placed the system in a freezer, the rate of freezing would be greater and all of the water would freeze. If the system were to stay at exactly 0 C, the ice would neither melt nor grow, because the rate of freezing would be equal to the rate of melting.
Freezing water undergoes the processes of nucleation and crystal growth. Nucleation is when large numbers of water molecules form clusters around a single point. This point is typically a microscopic particle, such as dust, which are naturally occurring in nondistilled water. Truly pure water will not actually freeze until below 32 F, due to lack of these microparticles During nucleation, the molecules are arranged into the shapes that the ice crystals will form. Once a cluster of cold water molecules forms, crystal growth begins, and ice forms.
When salt is added to water, it dissolves into sodium and chloride ions. These ions bond with the hydrogen and hydroxide ions that water is made of, forming small amounts of hydrochloric acid, or HCl, and sodium hydroxide, or NaOH. This process causes some of the water to be effectively removed from the system, as it is now part of a different chemical compound and, therefore, not available for the freezing process. Because there is less usable water, the rate of freezing decreases, and ice in the system will begin to melt. In order to reattain equilibrium, the temperature must be lowered to offset the change in rates. The addition of salt to water therefore lowers the freezing point, by removing some water from the freezing process.
The effect of lowered freezing temperature is useful in some practical applications. For example, ice cream was traditionally made by taking advantage of the rapid cooling effect of salted water. If you add salt to ice, the ice will melt, but the water will still be below 0 C. Because water can conduct temperature easier than ice can, you can then use the super-cooled water to cool down a container of cream rapidly, which will crystallize into ice cream. Salt in our oceans prevents them from freezing, allowing fish and other aquatic life to continue living during cold seasons. Many industrial applications use salt water instead of regular water to cool machinery, because the salt water can be colder than unsalted water.
Metal salts other than sodium chloride have the same effect on water but can render water toxic. This is because practically all salts are formed of ions that are capable of bonding with hydrogen and hydroxide. Common examples include calcium chloride, sodium bisulfate and potassium dichromate. Salted water can be reverted to regular water by the process of desalination. This process is used to create drinkable water on nautical vessels.